1. Field of the Disclosure
This disclosure relates to a method and apparatus of the logging of earth boreholes and particularly to method and apparatus for providing an indication of the earth formation porosity that is substantially independent of environmental factors such as mineralogy, borehole size, tool standoff, salinity, temperature, pressure etc.
2. The Related Art
It is well known in the prior art to utilize a neutron source and a pair of spaced radiation detectors to provide an indication of the formation porosity. With such prior art devices, the neutron source can be either a chemical source (such as an Am—Be source) or a pulsed neutron generator. The detectors can be either thermal neutron detectors or epithermal neutron detectors. Generally, it is the ratio of the count rates from the near and far detectors (the near-to-far ratio) that is indicative of the formation porosity. However, it is well-know that environmental factors, such as mineralogy, borehole size, tool standoff, tool standoff, salinity, temperature, pressure in borehole and formation etc., can affect the ratio and therefore the porosity measurement. U.S. Pat. No. 4,137,450 to Hopkinson teaches the use of different gates for providing an improved estimate of formation porosity in the presence of formation salinity when a pulsed neutron source is used. Other environmental effects, such as mineralogy, tool standoff and borehole size, etc. have not been addressed. The present disclosure teaches a method and apparatus of obtaining an improved estimate of formation porosity in real time while drilling that simultaneously accounts for these environmental factors. The method and apparatus are also capable of dealing with other environmental factors. Similar techniques can also be used in the formation density measurement to account for the environmental factors.